1. Field of the Invention
This invention relates to electrochemical reactors for conducting reduction and oxidation reactions in respective positive and negative liquid electrolytes, without gas evolution at the electrodes. More specifically the invention relates to a membrane-separated, bipolar multicell electrochemical reactor for implementing a redox flow battery system.
2. Description of Related Art
Redox flow battery systems are increasingly attracting interest as efficient energy conversion systems. Among redox couple candidates, the all vanadium redox system being one of the most preferred.
Structurally, the electrochemical reactors that have been proposed for redox flow battery systems, have been derived from the electrochemical reactor structures developed for general electrolysis processes, the only adaptation having concerned the materials employed as electrodes.
Generally, the electrochemical reactors used as redox batteries are conventionally composed of a stack of bipolar plate electrode elements separated by ion exchange membranes, defining a positive electrolyte flow chamber on one side of each membrane and a negative electrolyte flow chamber on the opposite side thereof. The stack of bipolar elements is assembled together in a filter-pass arrangement between two end electrode elements.
Commonly, the elements have a frame provided with co-ordinated through holes forming inlet and outlet manifolds for the two electrolytes that are circulated in a parallel mode through the positive electrolyte flow chambers and the negative electrolyte flow chambers, respectively.
The elements are conventionally mounted and operated in a vertical position.
The assembling of a large number of bipolar elements in electrical series as required in redox batteries to reach an adequate voltage at the two ends of the battery, the positioning of innumerable gaskets for sealing the outer perimeter of each electrolyte flow chamber and the perimeter of the distinct through holes of the frames for defining the inlet and outlet manifolds for the two electrolytes and the final tightening of the filter press assembly by tie rods compressing the two end elements over the stack, are extremely delicate and time consuming operations that require particularly skilled technicians.
The parallel flow of the two electrolytes through the respective flow chambers poses serious problems in terms of minimization of so-called stray or by-pass electric currents in uninterrupted liquid veins of electrolyte, due to the fact that the electrolyte present in the manifolds offer innumerable paths for these by-pass or stray currents, driven by mutual voltage differences existing among the various bipolar elements functioning in electrical series between the two end electrodes on which the full battery voltage difference insists. By-pass or stray currents decrement the energy efficiency of the conversion system, but more seriously they cause severe corrosion phenomena on conductive parts (e.g.: carbon) because of extremely high half-cell voltages at the conductor surface.
On the other hand, the redox system require non-negligible electrolyte flow rates through the flow chambers of the reactor in order to maintain optimal half-cell reactions conditions at the electrodes and this requirement may imply the necessity of operating the bipolar electrochemical reactor at relatively high positive pressure.
Differently from conventional electrochemical processing, redox flow battery systems are intended also for uses on nonpolluting vehicles and power/weight ratio is an important parameter.
A main objective of this invention is to provide a membrane-separated bipolar multicell electrochemical reactor for half-cell reduction and oxidation reactions in respective positive and negative electrolytes, without gas evolution, with an architecture that makes it more easily assemblable by allowing to stack fully pre-assembled elements horizontally, one on top of the other, and suitable to be operated in the same horizontal orientation of the bipolar elements.
According to a fundamental aspect of the novel architecture of the invention, the multicell assembly is constituted by alternately stacking two types of pre-assembled elements, one being a bipolar electrode holding subassembly and the other being a membrane holding subassembly.
Of course, the alternate stack of elements is piled over a bottom end element and the stack is terminated by placing over the last membrane holding element a top end electrode element. The two end electrode elements are then compressed over the stack by tightening a plurality of tie rods, conventionally arranged around the perimeter of the stacked elements, according to a common practice in tightening a filter-press stack in a hydraulically sealed manner, by virtue of the gaskets operatively installed between the coupling faces of the frames of the various elements.
According to an essential aspect of the architecture of the invention, each bipolar plate electrode holding element and each ion exchange membrane separator holding element includes a substantially similar rectangular frame piece, made of an electrically nonconductive and chemically resistant material, typically of molded plastic material, having on its upper (assembly) face grooves for receiving O-ring type gasket means, and having through holes and recesses in coordinated locations disposed along two opposite sides of the rectangular frame forming, upon completion of the assembling, ducts for the separate circulation of the negative electrolyte and of the positive electrolyte through all the negative electrolyte flow chambers and all positive electrolyte flow chambers, respectively, in cascade. The negative electrolyte enters along a first side of a negative electrolyte flow chamber, flows through the chamber toward the opposite or second side thereof, exits the chamber, flows through the coordinated holes through the frame holding the electrode and through the frame holding the next membrane separator, reaching the level of the next negative electrolyte flow chamber and enters it from the same second side through which it exited from the previous negative electrolyte flow chamber and exits this next negative electrolyte flow chamber from the same first side it entered the previous negative electrolyte flow chamber, to flow through coordinated holes through the next pair of frames to the level of the next negative electrolyte flow chamber and so forth. The same flow path is arranged also for the positive electrolyte, either in a xe2x80x9ccountercurrentxe2x80x9d or in an xe2x80x9cequicurrentxe2x80x9d mode through the battery.
In practice, the bipolar electrochemical reactor does not have inlet and outlet manifolds for the two electrolytes, on the contrary, the electrolytes flow through the respective flow chambers in a zigzag path, that is essentially in hydraulic series or cascade mode instead than in hydraulic parallel mode.
In this way, by-pass current may only be xe2x80x9cdrivenxe2x80x9d by a voltage difference of about one-cell voltage and becomes practically negligible and above all it does not cause any corrosion on conductive parts.
The two types of pre-assembled elements are coordinately xe2x80x9ckeyedxe2x80x9d so as to prevent any error in correctly stacking them alternately one over the other and with a correct orientation and perfect mutual alignment to compose the bipolar battery.
Apart from the suitably shaped keying pins and sockets and the position of the through holes and of the slotted portions of communication with the flow chambers, the molded plastic frames are substantially identical for both types of elements.
Essentially, each frame has an inner flange portion, recessed from the bottom (assembly) face of the frame, that is the opposite face to the one that is provided with the grooves for accommodating O-ring gaskets around pass-through electrolyte ducting holes and around the outer seal perimeter of the chamber.
During the pre-assembling of the two types of elements, on this inner flange portion is accommodated a relatively narrow edge perimetral portion of either a bipolar plate electrode or of an ion exchange membrane separator.
The two types of frames may conveniently be made of a different color or tonality for an easy recognition of which is destined to accommodate a bipolar plate electrode or an ion exchange membrane.
The face of the flange portion on which the ion exchange membrane separator or the bipolar plate electrode is set, has a plurality of orderly spaced retention pins that project out of the surface of the flange.
The plate electrodes and the ion exchange membrane separators are distinctively from each other, provided with coordinated through holes into which the retention pins of the respective frame piece pass.
A retention counterflange of an electrically nonconductive and chemical resistant material, typically of the same material and color of the respective type of frame, has also a number of holes coordinated with the positions of the retention pins and is functionally mounted over the perimetral portion of the plate electrode or of the membrane separator, whichever belongs to the particular type of frame, placed on the recessed flange portion of the frame.
The retention counter flange is fixed in position by flattening with a hot tool the protruding portions of the retention pins thus permanently fixing the bipolar plate electrode or the ion exchange membrane separator in the central window of the respective frame.
The rectangular shape of the frame windows into which membranes and bipolar plate electrodes, cut or made to size, are fitted, minimizes any waste of valuable membrane and bipolar plate material. A tight fitting of the counterflange effectively seals the contour of the membrane or bipolar plate preventing electrolyte intermixing reducing the number of gaskets. Optionally spacer gaskets may be used when necessary, for example to mount a particularly thin membrane.
This arrangement permits to overturn the so pre-assembled elements without the risk that the fitted bipolar plate electrode or ion exchange membrane separator may fall off and therefore permits to easily dispose the O-ring gaskets on the opposite (upper) face of the frame suitably provided with accommodating grooves.
Each pre-assembled element of a first type, may be overturned, placed on top of the stack and O-ring gaskets may be placed in the respective grooves before placing the next pre-assembled element of the other type on top, ready to receive the O-ring gaskets thereon and another pre-assembled element of the first type, and so on until completing the stack.
The keying pins and sockets, besides obliging a correct alternate stacking and orientation of bipolar electrodes and membrane separators pre-assembled elements, impose also a correct orientation on the plane of the elements such that the though holes and slotted portions coordinate with each other realizing the zigzag serial flow path of the two electrolytes.
Preferably the battery stack is made up of an integer multiple number of blocks each constituted by four alternately coupled elements: two membrane elements and two bipolar elements the battery having an even number of cells. In this way each electrolyte will enter and exit the battery from the same side.
Often and most preferably, the electrode consists of a porous fabric or mat of carbon fibers in electrical continuity with a similar electrode structure on the opposite face of the conductive bipolar plate electrode for providing substantially three-dimensional electrode structures having a large active area, that extends for a considerable portion into the depth of the relative electrolyte flow chamber.
This arrangement, dictated by the need to increase the allowable rate of half-cell reaction process at the electrode, contrasts with the need to minimize the power absorbed by the motors that drive the pumps of the two electrolytes to flow them through the plurality of respective flow chambers at an adequate flow rate.
This problem is aggravated when passing from a parallel flow of the electrolyte through all the respective flow chambers from a common inlet manifold to common outlet manifold to a cascade flow from one chamber to the next and so forth from one end to the opposite end of the stack.
Although this cascade flow mode is extremely effective in eliminating any corrosion problems due to by-pass currents, it necessarily implies an augmented pressure drop through the battery of the two electrolytes.
According to an important optional feature of the battery architecture of the invention, useful in case of use of porous three-dimensional electrodes extending from the impervious electrically conductive bipolar plate, this increase of pressure drop due to the use of a series or cascaded flow path of the electrolytes through the respective pluralities of flow chambers when using porous mat electrodes encroaching into the flow chamber is practically eliminated while reducing or even eliminating any residual gap or unobstructed flow space between the porous electrode and the ion exchange membrane separator, which, may even be placed in contact with each other to minimize ohmic losses in the liquid electrolyte.
These apparently contradictory conditions are indeed accomplished according to the present invention by defining (cutting) in the porous electrode two orders of parallel flow channels, all the parallel spaced channels of each order extend from a common orthogonal base channel formed along the respective inlet or outlet side of the chamber and terminate short of reaching the base channel of the other order. Each order defines a comb-shaped flow distributing channelwork the parallel fingers of which interleave with the finger channels of the other order.
Practically, one comb-shaped channelwork has its base or manifolding channel running along a side of the chamber communicating with the inlet duct of the electrolyte into the chamber while the other specular comb-shaped channelwork has its parallel finger channels interleaved with the parallel finger channels of the first channelwork and has its base or manifolding channel running along the opposite side of the chamber communicating with the outlet electrolyte duct.
The interleaved finger flow channels or one order run parallel to each other, each terminating short of the manifolding base channel of the other order of interleaved parallel finger channels. Therefore, each inlet or xe2x80x9csourcexe2x80x9d flow channel is separated from the two adjacent outlet or xe2x80x9cdrainxe2x80x9d flow channels by a strip of a certain width of the three-dimensional porous electrode material, separating the parallel channels that may be eventually cut in it.
The interleaved orders of inlet and outlet electrolyte flow channels evenly distribute the electrolyte with a reduced pressure drop uniformly throughout the electrode area of the flow chamber, providing flow distribution channels throughout the mass of the three-dimensional porous electrode.
The pressure drop may be pre-arranged within a certain margin, by knowing the specific pressure drop of the electrolyte through the three-dimensional porous electrode material at a given flow rate and by designing the two orders of interleaved xe2x80x9csourcexe2x80x9d and xe2x80x9cdrainxe2x80x9d flow channels with an appropriate distance of separation from one another.
Besides outstandingly reducing the pressure drop suffered by the electrolyte flowing through the respective flow chambers in series from one end to the opposite end of the battery, this arrangement of interleaved xe2x80x9csourcexe2x80x9d and xe2x80x9cdrainxe2x80x9d channels, suitably cut through the thickness of the three-dimensional porous electrode, is found to outstandingly enhance the electrochemical performance of the battery because of a far more evenly distributed current density over the entire cell area of the battery.
The invention is more clearly defined in the annexed claims.